Inherited disorders of intrahepatic bile duct development are a major cause of morbidity and mortality in the pediatric population. In recent years, advances have been made toward understanding the genetic control of liver development, but relatively little is understood regarding the molecular regulation of bile duct development. Identification of Jag1 as the disease gene for Alagille syndrome (AGS), an autosomal dominant disorder characterized by bile duct paucity along with anomalies in other organ systems, has revealed a crucial role for Jag1 and the Notch pathway in bile duct development. Jag1 encodes a ligand in the Notch signaling pathway, which is involved in cell fate determination in many organ systems. Homozygous targeted disruption of Jag1 in a mouse model causes early embryonic lethality, and the heterozygous mutant mouse has no phenotype. We have generated a Jag1 conditional knockout mouse using the Cre/lox gene targeting system. This model will permit the systematic study of the role of Jag1 in the development of multiple organ systems at specified times during development. Preliminary studies have demonstrated expression of Jag1 in the primitive bile ducts, or ductal plate cells, which originate from hepatocyte precursors in developing mouse and human liver. We propose to selectively ablate Jag1 in the developing liver and bile ducts by breeding the Jag1-loxP mouse with a liver-specific Cre transgenic line. Using this mouse model, we will perform detailed studies of ductal plate remodeling and bile duct development in the absence of Jag 1. In addition, we will employ real time PCR and micro array analysis to identify unique downstream targets of the Notch pathway in bile duct development. As a complementary strategy, we propose to use a cell culture approach in which we will derive bipotential hepatoblast cell lines from embryonic wildtype and Jag1 null livers. We will assess their response to defined stimuli, and rescue the phenotype by transfecting with activated Notch